In general, with development of mobile communication, use patterns of and demands for a communication device of users have been diversified. Users wish to perform communication without time and space restriction. However, an output of a radio frequency signal from a base station is restricted and a base station is mounted in each area or region. A shadow area may exist due to the location of a base station or the topography of an area. In order to eliminate the shadow area, an optical replay capable of obtaining a predetermined effect at low cost has been provided. The optical relay is mounted in an area in which a radio frequency signal is not received or is weak, such as the inside of a building, the basement of a building, a subway, a tunnel or an apartment complex of a residential area.
An optical relay system is used to extend the coverage of a base station such that a service is provided to a shadow area which a signal of a base station is unlikely to reach due to special topography or geographic features between base stations configuring a wireless communication service network.
In an optical relay system, a donor unit and a remote unit are connected through an optical cable. The donor unit converts an RF signal received from a base station into an optical signal and transmits the optical signal to the remote unit. The remote unit restores the optical signal received from the donor unit into an RF signal and transmits the RF signal to a mobile terminal.
The donor unit of the optical relay system includes a frequency module to amplify or control a TX signal received from the base station in order to send the TX signal to an apparatus and amplify or control an RX signal received from the apparatus in order to send the RX signal to the base station, on a frequency-by-frequency basis. In addition, the donor unit receives a band-limited signal from each frequency module and combines signals of multiple bands into one signal by passing only a signal of a desired band and filtering out a signal of an undesired band.
In the related art, for band combination of the signals output from the frequency modules into one signal, a filter corresponding to each frequency band is necessary. As the number of signals, the bands of which will be combined, is increased, the number of filters is increased. The filter needs to be accurately connected to each frequency module according to the frequency band, but may not be erroneously connected to each frequency module, in which case an original signal may be damaged. If frequency bands to be combined partially overlap, the bands may not be combined only using the filter.